1. Field of the Invention
The invention relates generally to methods for making carbon nanotubes and, more particularly, to a method for manufacturing carbon nanotubes with open-ended structure.
2. Discussion of Related Art
Carbon nanotubes (also herein referred to as CNTs) were first observed and reported in an article by Iijima in 1991 (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). Typically, carbon nanotubes are very small tube-shaped structures essentially having a composition of a graphite sheet in a tubular form. Carbon nanotubes have interesting and potentially useful electrical and mechanical properties and offer potential for various uses in electronic devices. Carbon nanotubes also feature extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (i.e. length/diameter ratios) (greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). These features make carbon nanotubes ideal candidates for electron field emitters, light sources, lithium secondary batteries, hydrogen storage cells, transistors, and cathode ray tubes (CRTs).
Generally, carbon nanotubes can be classified into single-wall type and multi-wall type. A single-wall type carbon nanotube structure is composed of only one cylindrical graphitic layer. A multi-wall type carbon nanotube structure is composed of two or more nested cylindrical graphitic layers.
It is already known that, there are three methods for manufacturing carbon nanotubes. The first method is the arc discharge method, which was first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). Carbon nanotubes produced by the arc discharge method are generally single-wall type. The second method is the laser ablation method, which was reported in an article by T. W. Ebbesen et al., entitled “Large-scale Synthesis of Carbon Nanotubes” (Nature, Vol. 358, 1992, pp. 220). This method has a low yield and is difficult to control a length and a diameter of the resulted carbon nanotubes. The third method is the chemical vapor deposition (CVD) method, which was reported in an article by W. Z. Li, entitled “Large-scale Synthesis of Aligned Carbon Nanotubes” (Science, Vol. 274, 1996, pp. 1701). The CVD method is useful in synthesis of carbon nanotube array and is advantageous in mass production, improved length controllability, compatibility with conventional integrated circuit process, etc.
Carbon nanotubes may have two terminal morphologies, i.e., close-ended structure and open-ended structure. The close-ended carbon nanotubes have a terminal/tip capped with a fullerene semi-sphere. The open-ended carbon nanotubes have an uncovered terminal/tip with an internal cavity exposed. However, carbon nanotubes produced by conventional CVD methods are generally close-ended structure. In order to improve a field emission properties, or if it is intended to incorporate some substance into inner spaces of close-ended carbon nanotubes, the tips thereof must be removed or uncovered. Conventionally, an additional process, e.g., oxidation using an oxidative acid solution, is needed to remove the capped tips. However, the oxidation process not only removes the capped tips but also oxidizes (burns) the nanotubes in a wide range thereby the peculiar nanotube structure may be damaged or even destroyed.
What is needed, therefore, is a method for manufacturing carbon nanotubes with open-ended structures without damages to their particular nanotube structure.